How To Choose Resonant Frequency Of Piezo Elements?

[alexoest]

I am about to order some piezo elements for experiments, i.e. mounting on various places on the body, adding magnets for directly picking up string vibrations, etc. I intend to use them with traditional guitar amplifiers, perhaps with buffer/preamp.

When I look at what's available (e.g. at Digi-Key), it strikes me that the resonant frequency of the elements is way above the frequency of the strings (the piezos often range between 1 and 5 KHz, and the strings are between 80 and 330 Hz). I assume that the ideal choice would be a direct match in frequency. But since that is not available - at least not at a low price - would it be a good idea to choose the elements available with the lowest resonant frequency, even though they're still way higher than the frequency of the strings?

Or are there other aspects (e.g. impedance or capacitance) of the piezo elements that should matter more in my choice? If so, what impedance or capacitance values should I go for?

I want to order a selection of piezo elements of different size and characteristics. I expect the elements to vary in sound (that's part of the fun), but I'd rater avoid buying something completely useless. If anyone can give some advice helping me to choose wisely, I'd be grateful.

/Alex

[erikbojerik]

Dig a little deeper and for some of them, you can get PDFs of the datasheets (basically spec sheets) from the manufacturer. There are links on the Digitech pages for specific piezo buzzers.

From looking at some of these data sheets, it appears that the manufacturer defines resonant frequency as the frequency at which the impedence is a minimum. In a way, this is sort of like measuring the frequency response of a pickup...there is a frequency at which the pickup's output is a maxiumum, and that is typically taken as the resonant frequency.

This doesn't mean the piezo will give zero output at other frequencies outside the resonant frequency. Beyond that, I'm not sure how important frequency matching is, given the freqs that you'll play on a guitar are all over the place. And I have no idea what one could tweek on a piezo (from a manufacturing standpoint) that would alter the resonant freq in one direction or another (I guess size and mass would be factors).

They're cheap as dirt, so it doesn't hurt to get a few and mess with 'em.

I wonder what the resonant freq of a Ghost saddle is....

[mammoth guitars]

Something else to keep in mind. Your note range for open strings is 82-330Hz but the range of a 22 fret guitar is 82-1174Hz up to D6.

[alexoest]

Thanks for the explanations and good points. I'll buy a selection of different piezos and give them a try. This is going to be fun... I bought a simple acoustic piezo pickup a couple of weeks ago and I am just amazed at what you can do with it... I've stuck it to a harmonica, an acoustic, an electric, a ukulele and a saw. The latter was not a big success, as it picked up a terrible noise from the bow, but I imagine it can be filtered away. Still, my main focus is to experiment with various placements and combinations of the piezos on the body and neck of an electric guitar.

Btw, I am going to buy some pots for tone and volume control of the piezos. Initially, it will be a passive piezo-only configuration, so I don't have to take blending with traditional pickups into account. Suggestions for resistance values of the pots are very welcome - I've seached this forum and other sources, but only found information on suitable pot resistance for combining the piezos with traditional pickups.

/Alex

[erikbojerik]

Can't help you on the pot values....though I have an old Ovation with passive volume & tone with an undersaddle piezo. If I can find some time, I'll try to see if I can measure the resistance between tip & ground.

For the electric try sticking the piezo to the headstock....

[psw]

I put a piezo into the neck socket of a strat with good sonic results once, but I got a lot of handling and the noise of fretting the strings onto the neck as well. Although piezos can put out enough output to get a sound on their own, they really need a preamp or buffer to get the impedance to match that of an amp, not just to mix with magnetic pickups. A simple preamp can be had in kit form from a lot of electronics places cheap that will allow a better impedance match and provide a much better sound. A volume pot in this case would be what is recommended for the circuit usually 10k-50k...I imagine that a quite large value would be required without it but would add noise and loading effects.

BTW...almost every stompbox has at least a buffer in it, often on even when turned off (unless true bypass, etc) so you can hear the difference by running a piezo through a box on the way to the amp. Something fairly transparent like a compressor is good, or an active volume pedal. If you already run your experiments into a "box" like a pod or something, this will not be the same as the result running straight to an amp.

I am trying a similar thing with an LP type guitar I am building attaching a buzzer element under a kahler bridge on the wood itself to try and bring in a bit of acoustic edge to the electric sound. I'll let you know hos I get on with it...

pete

[alexoest]

Both: Thanks for taking time to help.

Erik,

Thanks for offering to measure the pot, but I think I'll just buy a couple of trimpots and try them out. I'd be nice to know the resistance, but unless it can be easily done, I don't think it's worth the trouble. I've just found that Graphtech recommends a 5 Mega Ohm pot, so I'll try that. Couldn't find anything on the resonant frequency or other specs of the Ghost saddles, btw.

I tried sticking my piezo-pickup on the headstock of my acoustic, but only the open strings sounded OK. The fretted ones were weak, especially when fretted high on the fingerboard. I'll try on an electric as it might be due to too much damping in the wood of my acoustic's neck.

Pete,

Re. tone and volume pots: how if I make a circuit of, say, four piezos, each one of which I want to be able to adjust for tone and volume to get them in balance. Wouldn't it be relevant to do that by means of trimpots and switches and then connect their common, balanced and adjusted output to the preamp?

I do have an old fuzz pedal lying around. I'll give that a try. Didn't know they had a buffer.

Sounds interesting with the element mounted in wood. I've heard that mounting it directly on the bridge can give quite a shrill tone.

Btw, you once mentioned the idea of mounting magnets on piezos to pick up string vibrations. Did you ever follow up on that idea? Sounds like a great way to pick up the strings' vibrations (with antinodes and nth harmonics unpolluted by body and neck vibrations) with a cheap and compact pickup.

/Alex

[psw]

Ooops, forgot to press the notification button...hence no reply...

Pete,

Re. tone and volume pots: how if I make a circuit of, say, four piezos, each one of which I want to be able to adjust for tone and volume to get them in balance. Wouldn't it be relevant to do that by means of trimpots and switches and then connect their common, balanced and adjusted output to the preamp?

I do have an old fuzz pedal lying around. I'll give that a try. Didn't know they had a buffer.

Sounds interesting with the element mounted in wood. I've heard that mounting it directly on the bridge can give quite a shrill tone.

Btw, you once mentioned the idea of mounting magnets on piezos to pick up string vibrations. Did you ever follow up on that idea? Sounds like a great way to pick up the strings' vibrations (with antinodes and nth harmonics unpolluted by body and neck vibrations) with a cheap and compact pickup.

/Alex

Yeah...well you know I'm full of...errr...ideas, that is!

The sustainer project has been so frustrating at time that I was prepared to do anything! So, since the driver acts in reverse to a pickup (vibrating the strings rather than picking up vibrations) and suffers from putting out a lot of EMI that will get into other electromagnetic devices (such as conventional coil pickups) then one idea was to try and move the strings by moving small neodyminium magnets under them with piezos. Anything like this with the sustainer project should work (for better or worse ) in reverse as a pickup; collecting rather than transmitting signals. It was a bit of a dead ender of an idea and was not really pursued seriously. For one thing, the moving magnets would still produce fluctuating magnetism (creating EMI anyway) and it is effectively a mechanical system, so there are going to be lots of inefficiencies...intereting notion though and a good lateral thinking exercise. As a side note, a piezo pickup and electromagnetic driver should avoid EMI problems with a sustainer...so perhaps on a Variax, if line6 would like to contact me and give me one for research purposes )

Sounds like a great way to pick up the strings' vibrations (with antinodes and nth harmonics unpolluted by body and neck vibrations) with a cheap and compact pickup.

I am not sure at all about this and you may need to meditate deeper into what you are suggesting. For one thing, the presence of magnets under the strings is likely to effect the vibration. The pickup itself would still have to be connected to something...presumably the body and so the pickup will vibrate and effect what it hears. Piezos are extremely non-liniear and their resonant frequencies and such do not suggest a "pure" picking up of vibrations, their characteristics generally are not even those that our ear likes to hear, which is more the case with magnetic pickups.

You may also consider the characterizing of the neck and body wood vibrations as pollution...especially here at PG. A lot of people put a lot of work and some fetishism of body materials to obtain a nice "polluted" mix in their constructions. I am not even sure a very neutral sound (say a stiff carbon fibre steinberger with low magnetic EMG's) is even something that most would seek! However, cheap and compact pickup....now there is something I guess, but a magnet and a coil of wire is fairly easily obtainable and well tested; it is unlikely that a piezo system would ever be satisfactory without a preamp and probably filtering and of course a battery, at least with normal amplifiers designed for high impedance pickups.

I do have an old fuzz pedal lying around. I'll give that a try. Didn't know they had a buffer.

Some of the old pedals don't...a boss probably does. The fuzz will be a preamp when on, but will likely effect the sound adversely if it can't be turned down to no effect...a fuzz is a distorting preamp essentially.

Sounds interesting with the element mounted in wood. I've heard that mounting it directly on the bridge can give quite a shrill tone.

A piezo will pickup anything..even your hand on the bridge or the knocking of your fingers as they fret the strings if mounted in the neck. So, they can be noisy in use, whereas a magnetic pickup will only pickup the vibrations of electromagnetic materials or radiated signals. Some acoustics with saddle piezos for instance will pickup the overenthusiastic damping on the bridge creating a thumping with some techniques. Their higher resonant frequency response often picks up the click of the pick a lot more too!

Generally, piezos as you find on an electro acoustic or electric saddle system require some tailoring of the sound to filter out such effects...hence you find a lot of EQ controls in the preamps or recommend a separate amplifier that can be tailored to the piezos without effecting the electric sound coming out of it's own amps.

All piezo's then can sound shrill or quacky, but preamps can be used to lessen this effect. One thing piezos do very welll is give a very quick response while a magnetic pickup often has more of a bloom, especially humbuckers it would seem.

SO...if they pickup anything that vibrates them, it need not necessarily have to be attached directly to the strings themselves via the bridge saddles. This gives the strongest response, but primarily picks up the tone of the strings vibration (polluted as it is by the guitar construction it is mounted on). SO, high end acoustic systems often have body transduces to pickup the sound of the top, I think tailor has one in the neck pocket of some too, others use internal mics too. Of course a true acoustic officianado will only use high quality external mics to pickup the true sound of the entire instrument.

So, yes...a bridge mounted system will often sound shrill and require filtering to tone it down. It still lacks the "body" or "woody" sound of a mic'ed instrument. On an electric, there is not a lot of body sound and it is not really designed for vibrating as such, more for sustain (in general) and so can sound a little shrill if you could here it.

With some trem mounted piezos, it will pickup the squeak of the springs, although many have had success. My Kahler trems are extremely solid and mounting piezos would be problematic. The do however mount extremely solidly to the guitar with a lot of tone transfer (they are not a fulcrum suspended on springs like a fender or floyd that will absorb and change the manner of string vibration transfer to the guitar). Someone emailed me with an idea similar and although I have tried to simplify my latest projects (look out for future threads on this as they reach completion) I was tempted to give it a go on this particular guitar to see how it would go.

On my LP then, I have mounted a piezo directly to the top of the guitar directly under the bridge, however, one under the bridge pickup would probably be close enough. I expect that the sound alone, even with a basic preamp, may be a little short on performance alone, the idea is purely to mix it with the electric tone. It is the combination of a full humbucker type tone with an immediate attack and perhaps some "woody" tone that is what tempted me with this particular project. An electric guitar is never really going to sound "acoustic", nor would I really desire that (although a lot of parkers and ghost guitars do a pretty good imitation...however, I have heard some pretty good acoustic simulators lately that require no novel preamps at all).

I encourage you with your experiments, but really preamps are a necessary evil. They will last up to a year though and need not require a lot of battery changing or anything. As an addition to magnetic pickups, I think they could be very interesting but as a "pseudo acoustic", for me a little less so. Technology is coming along and the Variax indicates a good use of piezos with extreme digital modeling and even synthesizer and computer interface applications that may eventually see the end of magnetic pickups as we use them today.

Re. tone and volume pots: how if I make a circuit of, say, four piezos, each one of which I want to be able to adjust for tone and volume to get them in balance. Wouldn't it be relevant to do that by means of trimpots and switches and then connect their common, balanced and adjusted output to the preamp?

Well this is ok, but not as good as a separate mixer preamp before combining them. Each pot and tone will effectively load them down even more and take up a lot more room that a single chip preamp that could have 4 separate op-amps on a compact $2 chip. If you are going to go preamps (which realistically you must IMHO) then this is probably the end result. However, for starters a simple clean preamp would be plenty and result in any number of piezos being adjusted or buffered to a workable impedance and tone down the shrillness and other unwanted effects of a passive piezo. Remember too that the phase and positioning of piezos will also cancel and reinforce frequencies and responses...something that could be used to your advantage, or cause problems. Turn them upside down, reverse wires in respect to one another or just move them about the body till it sounds "right" to you is my advice...have fun and have an open mind.

What we aim for may not be what we get or perhaps idealize, but it may be even better. The cheapo buzzer DIY piezos have at times been a little maligned, however they will produce a sound for very little expense and may well compliment a conventional electric depending largely on it's application. I still have hope that mine will produce an interesting effect and I have in the past had some amazing sounds with distortion and such with combinations as the quick attack of the things sounds like a sledgehammer...super crisp and crunchy attack! My secret placement of the device in this project should create no bridge handling noise and I am not expecting too much of it that it would ever work other than to alter the sound of the inherent electric guitar that it is.

Will let you know when the project is closer, need some more parts and work and time (need to cut a nut, etc) before it can really be tested...I took a pic while fitting it and if it should be "amazing" I will disclose all!

pete

[alexoest]

Pete,

My idea with the string-sensing piezo was that mounting the piezo element on rubber or foam would minimize the vibrations of the body. I put two neodymium magnets on my acoustic-pickup and held them under two of the strings on an electric guitar. It worked and even sounded good, though it did have a tendency to pull at the strings, but I believe that can be solved by using a magnet of the appropriate strength.

Referring to the guitar body's vibration as pollution was not meant in a negative way, BTW. One of the thing I look forward to with the piezos is to be able to pick up that vibration more efficiently than with a traditional pickup in a pickguard. Still, if you want only specific antinodes on the strings to be picked up, the rest _is_ sort of pollution.

The preamp you describe with several separate op-amps sounds useful for my purpose - and indeed for anyone who wants to use several piezos in one guitar. I've had a look at various preamp schematics (and I'll get some parts for assembling one or two now that I am ordering the piezo elements anyway). Still, I am a novice at electronics, so modifying an existing design to be able to adjust several inputs is not something I will attempt. Not at the moment, anyway. But if you know of an existing design (assembled or as a schematic) like the one you describe, I'd be happy to know.

Please disclose your the experiments you mention, including the secret placement of the piezo element, even if the results are less than amazing! And again: Thanks for the advice, which I am sure has saved me a lot of frustration. I'm convinced and I'll buy - or make - a preamp. I look forward to a lot of fun - There are so many options to be explored. I'll have to control my urge to do everything at once.

/Alex

[psw]

Hey Alex...no don't give up on these ideas. I do understand what you are meaning my "pollution"...and you have clarified it further, anything not wanted in the signal is pollution. I was trying to add caution to idealizing something that is a whole system and being unrealistic in what is aimed for.

As far as preamps go, check out the electronics places for kits...I have used the "prechamp" kit quite a lot and it can be adapted for varying gain. It is reasonably small and easy to put together and cheap...also it was designed by "silicon chip" magazine in Australia as it happens, so for me, readily available.

The prechamp is fairly small, uses a two transistor circuit and offers filtering of frequencies outside of the audio range and the instructions also explain the way in which this works (it is an educational kit as well). Knowing that this particular cap and resistor are being used to filter these frequencies encourages one to perhaps subsititute different components for different effects, for instance. In Australia it sells for about A$7 so don't pay too much! A 10k pot should work as a volume control after it as this is what is used on the original LM386 based "champ" power amp circuit that it was designed for.

High impedance input, low impedance output. But the same is true of op-amp based circuits using chips like the 071 series of chips. Often the data sheets contain circuits so if you look up the chip (say 071 data sheet) in google you may find something to design your own. Better results are achieved quicker by using a kit though...more time experimenting with the piezos!

Also, llok out for piezos in all kinds of devices. I have found very cheap ones inside of these widow buzzer alarms for less than A$2 and once broken open you get a switch and a reed switch (that I use on my coil winder counters).

Beware the use of neodyminium magnets...they do have a strong field that will effect the vibration of the strings adversely. Never try to cut them or file them in anyway as this is extremely dangerous.

I think the magnetic thing is a little bit of a "furphy" (something only Australians might understand) or dead end, that will not really achieve the results desired. Better to start by attaching piezos in various locations to get a mix of sounds that together achieves the result. Also, you could try reversing them as I think I mentioned which may achieve some cancelling effects. Be aware also of the potential for handling noise and keep an eye on practicality too. I dare say there is plenty to explore just using masking tape and attaching them all over the body of various guitars and noting the effect. Perhaps one day someone will make a giant piezo element that could be used for the entire top of a guitar...mwaahahahha....

Have fun, play safe and let us all know how you go. Feel free to email me for anything specific if you think I can help. I am not an electronics genius either by the way, so don't ask me to design anything, but I may have some clues for stuff I have seen or have heard of, or even built along the way...

Good Luck... pete

[alexoest]

The Prechamp sounds like a good place to start, especially so since it is so well documented. Still, to get started without too many potential pitfalls, I'll buy a finished preamp and wait making one myself until I know more.

BTW, the giant piezo element covering a whole guitar top might actually be possible with piezoelectric paint. Painting the inside of a hollowbody with piezo paint would be one way to do it.

You can do that... I'll play around with the smaller piezo elements in the meantime ;-)

/Alex